The present invention relates to the field of relay protection for an electric power system, and in particular, to a frequency-conversion differential protection method for an output transformer of a static frequency converter (SFC) system and a corresponding relay protection device or monitoring device.

An SFC system is generally applied to a pumped storage unit and a large gas turbine generator unit, to implement frequency-conversion start-up of the unit. Currently, domestic SFC systems are all devices imported from abroad. The relay protection of an SFC system is generally integrated in the SFC regulator. Frequencies of currents on high and low voltage sides of an output transformer of the SFC system are variable, and differential protection algorithms for conventional transformers are all based on power frequency current; therefore, the relay protection function is difficult to be applied to the output transformer. In addition, none of foreign SFC systems is configured with a differential protection function for an output transformer, and instantaneous overcurrent protection serves as primary quick protection; as a result, the sensitivity is low and it is harmful to a protected device.

An objective of the present invention is to provide a frequency-conversion differential protection method for an output transformer of an SFC system, which uses a generator start-up and shutdown protection algorithm which unaffected by frequency to calculate amplitude values of the differential current and the restraint current of the output transformer, so as to implement biased differential protection, adapt to a large-scale frequency variation, and improve the sensitivity of detection on an internal fault of the output transformer.

A technical solution used by the present invention is described as follows: In a frequency-conversion differential protection method for an output transformer of an SFC system, a protection device of the SFC system measures three-phase current on each side of the output transformer of the SFC system; phase correction and balance coefficient adjustment are performed on the current on each side of the output transformer according to the connection manner of the output transformer, to calculate sampling values of correcting current on each side and the differential current; a generator start-up and shutdown protection algorithm which unaffected by frequency is used to calculate amplitude values of the correcting current, the differential current, and the restraint current; and frequency-conversion differential protection for the output transformer is implemented by using biased differential characteristic and according to magnitudes of the differential current and the restraint current.

The output transformer includes a two-winding transformer or a three-winding transformer; according to the connection manner of the output transformer, triangle side is used as a reference to perform phase correction on the star side; and at the same time, in consideration of the secondary rated current on each side of the transformer are different, a balance coefficient on each side is adjusted, so as to calculate the sampling values of the correcting current on each side of the output transformer and the differential current.

The generator start-up and shutdown protection algorithm which unaffected by frequency is used to calculate the amplitude values of the correcting current, the differential current, and the restraint current, so as to adapt to a large-scale frequency variation, where the start-up and shutdown protection algorithm includes a zero crossing point integral algorithm and a current peak value determining method.

A method for calculating the balance coefficient on each side is the same as that in differential protection of a conventional transformer, and balance coefficient on high voltage side is set to K_H, balance coefficient on low voltage side 1 is set to K_L1, and balance coefficient on low voltage side 2 is set to K_L2; i_Ha.j(k), i_Hb.j(k), i_Hc.j(k), i_L1a.j(k), i_L1b.j(k), i_L1c.j(k), i_L2a.j(k), i_L2b.j(k) and i_L2c.j(k) are calculated, which separately are sampling values of three-phase correcting currents on the high voltage side, the low voltage side 1 and the low voltage side 2; and three-phase differential currents are calculated by using the correcting current on the high and low voltage sides: $$\{\begin{array}{c}{i}_{\mathit{da}}\left(k\right)=i_{\mathit{Ha}\mathrm{.}j}\left(k\right)+i_{L1a\mathrm{.}j}\left(k\right)+i_{L2a\mathrm{.}j}\left(k\right)\\ i_{\mathit{db}}\left(k\right)=i_{\mathit{Hb}\mathrm{.}j}\left(k\right)+i_{L1b\mathrm{.}j}\left(k\right)+i_{L2b\mathrm{.}j}\left(k\right)\\ i_{\mathit{dc}}\left(k\right)=i_{\mathit{Hc}\mathrm{.}j}\left(k\right)+i_{L1c\mathrm{.}j}\left(k\right)+i_{L2c\mathrm{.}j}\left(k\right)\end{array}$$
where i_da(k), i_db(k), and i_dc(k) separately are sampling values of the three-phase differential currents.

By using the generator start-up and shutdown protection algorithm which unaffected by frequency, amplitude values (I_Ha.j, I_Hb.j, and I_Hc.j) of the correcting currents on the high voltage side are calculated by using the sampling values of the correcting currents on the high voltage side; amplitude values (I_L1a.j, I_L1b.j, and I_L1c.j) of the correcting currents on the low voltage side 1 are calculated by using the sampling values of the correcting currents on the low voltage side 1; amplitude values (I_L2a.j, I_L2b.j, and I_L2c.j) of the correcting currents on the low voltage side 2 are calculated by using the sampling values of the correcting currents on the low voltage side 2; and amplitude values (I_da, I_db, and I_dc) of the differential currents are calculated by using the sampling values of the differential currents.

Restraint currents are calculated by using the correcting currents on the high and low voltage sides as follows: $$\{\begin{array}{c}{\mathit{I}}_{\mathit{ra}}=\frac{{\mathit{I}}_{\mathit{Ha}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{1}\mathit{a}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{2}\mathit{a}\mathrm{.}\mathit{j}}}{2}\\ {\mathit{I}}_{\mathit{rb}}=\frac{{\mathit{I}}_{\mathit{Hb}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{1}\mathit{b}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{2}\mathit{b}\mathrm{.}\mathit{j}}}{2}\\ {\mathit{I}}_{\mathit{rc}}=\frac{{\mathit{I}}_{\mathit{Hc}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{1}\mathit{c}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{2}\mathit{c}\mathrm{.}\mathit{j}}}{2}\end{array}$$
where I_ra, I_rb, and I_rc separately are three-phase restraint currents.

Frequency-conversion differential protection for the output transformer is implemented by using the biased differential characteristic and according to the magnitudes of the differential current and the restraint current, where the biased differential characteristic may be a bilinear or multi-linear biased characteristic, a dual-slope biased characteristic, or a variable-slope biased characteristic.

A differential protection operation equation of a bilinear biased characteristic is as follows: $$\{\begin{array}{cc}{I}_d>I_s\hfill & {\mathit{when\; I}}_r<I_t\\ I_d>I_s+k\cdot \left(I_r-I_t\right)\hfill & {\mathit{when\; I}}_r\ge I_t\end{array}$$
where I_r is the restraint current, I_d is the differential current, I_s is the differential protection start-up current, It is the knee current, and k is the slope coefficient; and
split-phase determining is used, and when the operation equation of the formula (3) is met, the differential protection of the output transformeroperate.

Beneficial effects of the present invention are described as follows: frequency-conversion differential protection for the output transformer of the SFC system is implemented, and the protection method in the present invention adapts to the large-scale frequency variation, and compared with instantaneous overcurrent protection, greatly improves the sensitivity of detection on an internal fault of the output transformer, and better ensures the security of a device.

• FIG. 1 is a schematic diagram of an SFC system and current measurement on each side of an output transformer according to the present invention, where TR1 is an input transformer, TR2 is the output transformer, Ld is a direct current reactor, NB1 is a network bridge side rectifier bridge 1, NB2 is a network bridge side rectifier bridge 2, MB1 is a machine bridge side inverter bridge 1, MB2 is a machine bridge side inverter bridge 2, VCB1 is an input circuit breaker, VCB2 is an output circuit breaker, S1 is an isolation switch on a low voltage side of the output transformer, S2 is an isolation switch at a bypass, CT1 and CT2 separately are current transformers on low voltage side 1 and 2 of the output transformer, and CT3 is a current transformer on a high voltage side of the output transformer;
• FIG. 2 is a diagram of a bilinear biased characteristic, where I_r is the restraint current, I_d is the differential current, I_s is the differential protection start-up current, I_t is the knee current, and k is the slope coefficient;
• FIG. 3 is a diagram of a dual-slope biased characteristic, where I_r is the restraint current, I_d is the differential current, I_s is the differential protection start-up current, k₁ and k₂ respectively are the slope coefficient 1 and slope coefficient 2, I_t1 and I_t2 are the knee current 1 and knee current 2, and a fitting curve exists between the two knee currents; and
• FIG. 4 is a diagram of a variable-slope biased characteristic, where I_r is the restraint current, I_d is the differential current, I_s is the differential protection start-up current, k₁ and k₂ respectively are the initial slope and maximum slope, I_t is the knee current, and a variable-slope fitting curve exists between the initial slope and the maximum slope.

The following describes the technical solutions of the present invention in detail with reference to the accompanying drawings.

In FIG. 1, three-phase currents on all sides are measured from CTs on high and low voltage sides of an output transformer, and sampling values of the correcting current and the differential current on each side of the output transformer are obtained after phase correction and balance coefficient adjustment are performed on the current on each side of the output transformer according to the connection manner of the output transformer.

According to the connection manner of the output transformer, triangle side is used as a reference to perform phase correction on star side; at the same time, in consideration of the secondary rated current on each side of the transformer are different, a balance coefficient on each side is adjusted, so as to calculate the sampling values of the correcting current on each side and the differential current. Using a Y, d, d-11 connection manner as an example, the positive direction is defined as the current on each side flow into the transformer, sampling values of three-phase currents on high voltage side of the output transformer are set to i_Ha(k), i_Hb(k), and i_Hc(k), sampling values of three-phase currents on the low voltage side 1 are set to i_L1a(k), i_L1b(k), and i_L1c(k), and sampling values of three-phase currents on the low voltage side 2 are set to i_L2a(k), i_L2b(k), and i_L2c(k), so as to perform phase correction on the three-phase currents on the high voltage side as the following formula: $$\{\begin{array}{c}{i}_{\mathit{Ha}}^{ʹ}\left(k\right)=\frac{i_{\mathit{Ha}}\left(k\right)-i_{\mathit{Hb}}\left(k\right)}{\sqrt{3}}\\ i_{\mathit{Hb}}^{ʹ}\left(k\right)=\frac{i_{\mathit{Hb}}\left(k\right)-i_{\mathit{Hc}}\left(k\right)}{\sqrt{3}}\\ i_{\mathit{Hc}}^{ʹ}\left(k\right)=\frac{i_{\mathit{Hc}}\left(k\right)-i_{\mathit{Ha}}\left(k\right)}{\sqrt{3}}\end{array}$$
where i'_Ha(k), i'_Hb(k), and i'_Hc (k) are sampling values of the three-phase currents after phase correction is performed on the high voltage side.

A method for calculating the balance coefficient on each side is the same as that in differential protection of conventional transformer, and balance coefficient on high voltage side is set to K_H, balance coefficient on low voltage side 1 is set to K_L1, and balance coefficient on low voltage side 2 is set to K_L2, so that a computing formula of the sampling values of the correcting currents on the high and low voltage sides is as follows: $$\{\begin{array}{c}{i}_{\mathit{Ha}}\left(k\right)=i_{\mathit{Ha}}^{ʹ}\left(k\right)\cdot K_H\hfill \\ i_{\mathit{Hb}}\left(k\right)=i_{\mathit{Hb}}^{ʹ}\left(k\right)\cdot K_H\hfill \\ i_{\mathit{Hc}}\left(k\right)=i_{\mathit{Hc}}^{ʹ}\left(k\right)\cdot K_H\hfill \\ i_{L1a\mathrm{.}j}\left(k\right)=i_{L1a}\left(k\right)\cdot K_{L1}\hfill \\ i_{L1b\mathrm{.}j}\left(k\right)=i_{L1b}\left(k\right)\cdot K_{L1}\hfill \\ i_{L1c\mathrm{.}j}\left(k\right)=i_{L1c}\left(k\right)\cdot K_{L1}\hfill \\ i_{L2a\mathrm{.}j}\left(k\right)=i_{L2a}\left(k\right)\cdot K_{L2}\hfill \\ i_{L2b\mathrm{.}j}\left(k\right)=i_{L2b}\left(k\right)\cdot K_{L2}\hfill \\ i_{L2c\mathrm{.}j}\left(k\right)=i_{L2c}\left(k\right)\cdot K_{L2}\hfill \end{array}$$
where i_Ha.j(k), i_Hb.j(k), and i_Hc.j(k) separately are sampling values of three-phase correcting currents on the high voltage side, i_L1a.j (k), i_L1b.j(k), and i_L1c.j (k) separately are sampling values of three-phase correcting currents on the low voltage side 1; and i_L2a.j(k), i_L2b.j(k), and i_L2c.j(k) separately are sampling values of three-phase correcting currents on the low voltage side 2.

Three-phase differential currents are calculated by using the correcting current on the high and low voltage sides: $$\begin{array}{c}{i}_{\mathit{da}}\left(k\right)=i_{\mathit{Ha}\mathrm{.}j}\left(k\right)+i_{L1a\mathrm{.}j}\left(k\right)+i_{L2a\mathrm{.}j}\left(k\right)\\ i_{\mathit{db}}\left(k\right)=i_{\mathit{Hb}\mathrm{.}j}\left(k\right)+i_{L1b\mathrm{.}j}\left(k\right)+i_{L2b\mathrm{.}j}\left(k\right)\\ i_{\mathit{dc}}\left(k\right)=i_{\mathit{Hc}\mathrm{.}j}\left(k\right)+i_{L1c\mathrm{.}j}\left(k\right)+i_{L2c\mathrm{.}j}\left(k\right)\end{array}$$
where i_da(k), i_db(k), and i_dc(k) separately are sampling values of the three-phase differential currents.

By using start-up and shutdown protection algorithm (for a zero crossing point integral algorithm and a current peak value determining method, see "Chen Deshu. Principle and Technology for Computer Relay Protection [M]. Beijing: China Electric Power Press, 1992.") which unaffected by frequency and maturely applied in generator protection, amplitude values (I_Ha.j, I_Hb.j, and I_Hc.j) of the correcting currents on the high voltage side are calculated by using the sampling values of the correcting currents on the high voltage side; amplitude values (I_L1a.j, I_L1b.j, and I_L1c.j I_L1c._j) of the correcting currents on the low voltage side 1 are calculated by using the sampling values of the correcting currents on the low voltage side 1; amplitude values (I_L2a.j, I_L2b.j, and I_L2c.j) of the correcting currents on the low voltage side 2 are calculated by using the sampling values of the correcting currents on the low voltage side 2; and amplitude values (I_da, I_db, and I_dc) of the differential currents are calculated by using the sampling values of the differential currents.

Restraint currents are calculated by using the correcting currents on the high and low voltage sides as follows: $$\{\begin{array}{c}{\mathit{I}}_{\mathit{ra}}=\frac{{\mathit{I}}_{\mathit{Ha}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{1}\mathit{a}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{2}\mathit{a}\mathrm{.}\mathit{j}}}{2}\\ {\mathit{I}}_{\mathit{rb}}=\frac{{\mathit{I}}_{\mathit{Hb}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{1}\mathit{b}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{2}\mathit{b}\mathrm{.}\mathit{j}}}{2}\\ {\mathit{I}}_{\mathit{rc}}=\frac{{\mathit{I}}_{\mathit{Hc}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{1}\mathit{c}\mathrm{.}\mathit{j}}+{\mathit{I}}_{\mathit{L}\mathrm{2}\mathit{c}\mathrm{.}\mathit{j}}}{2}\end{array}$$
where I_ra, I_rb, and I_rc separately are three-phase restraint currents.

Frequency-conversion differential protection for the output transformer is implemented by using the biased differential characteristic and according to the magnitudes of the differential current and the restraint current. The biased differential characteristic may be a bilinear or multi-linear biased characteristic, a dual-slope biased characteristic, or a variable-slope biased characteristic (see "Gao Chunru. Setting Calculation and Operation Technology of Relay Protection for Large Generator Unit (second edition) [M]. Beijing: China Electric Power Press, 2010.") shown in FIG. 2, FIG. 3, and FIG. 4.

Using the bilinear biased characteristic shown in FIG. 2 as an example, a differential protection operation equation is as follows: $$\{\begin{array}{cc}{I}_d>I_s\hfill & {\mathit{when\; I}}_r<I_t\\ I_d>I_s+k\cdot \left(I_r-I_t\right)\hfill & {\mathit{when\; I}}_r\ge I_t\end{array}$$
where I_r is the restraint current, I_d is the differential current, I_s is a differential protection start-up current, It is a knee current, and k is a slope coefficient.

Split-phase determining is used, and when the operation equation of the formula (5) is met, differential protection of the output transformer operate.

The foregoing embodiments are merely used for describing the technical ideas of the present invention rather than limiting the protection scope of the present invention, and any variation made based on the technical solutions according to the technical ideas provided by the present invention shall fall within the protection scope of the present invention.